In rechargeable electrochemical battery cells, weight and portability are important considerations. It is also advantageous for rechargeable battery cells to have long operating lives without the necessity of periodic maintenance. Rechargeable electrochemical battery cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable electrochemical cells can also be configured as larger “cell packs” or “battery packs”.
Rechargeable electrochemical battery cells may be classified as “nonaqueous” cells or “aqueous” cells. An example of a nonaqueous electrochemical battery cell is a lithium-ion cell, which uses intercalation compounds for both anode and cathode, and a liquid organic or polymer electrolyte. Aqueous electrochemical cells may be classified as either “acidic” or “alkaline”. An example of an acidic electrochemical battery cell is a lead-acid cell, which uses lead dioxide as the active material of the positive electrode and metallic lead, in a high-surface area porous structure, as the negative active material. Many of the alkaline electrochemical battery cells are nickel based. Examples of such cells are nickel cadmium cells (NiCd), nickel metal hydride cells (NiMH), nickel hydrogen cells (NiH), nickel zinc cells (NiZn), and nickel iron cells (NiFe).
Ni—MH cells use negative electrodes having a hydrogen absorbing alloy as the active material. The hydrogen absorbing alloy is capable of the reversible electrochemical storage of hydrogen. Ni—MH cells typically use a positive electrode having nickel hydroxide as the active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte such as potassium hydroxide.
Upon application of an electrical current across a NiMH cell, water is dissociated into a hydroxyl ion and a hydrogen ion at the surface of the negative electrode. The hydrogen ion combines with one electron and forms atomic hydrogen and diffuses into the bulk of the hydrogen storage alloy. This reaction is reversible. Upon discharge, the stored hydrogen is released to form a hydrogen ion and an electron. The hydrogen ion combines with a hydroxyl ion to form water. This is shown in equation (1):
                    M        +                              H            2                    ⁢          O                +                              e            -                    ⁢                                          ⁢                      ⟷            discharge            charge                    ⁢                                          ⁢          M—H                +                  OH          -                                    (        1        )            
The reactions that take place at the nickel hydroxide positive electrode of a Ni—MH battery cell are shown in equation (2):
                                          Ni            ⁡                          (              OH              )                                2                +                              OH            -                    ⁢                                          ⁢                      ⟷            Discharge            charge                    ⁢                                          ⁢          NiOOH                +                              H            2                    ⁢          O                +                  e          -                                    (        2        )            
The use of disordered negative electrode metal hydride material significantly increases the reversible hydrogen storage characteristics required for efficient and economical electrochemical cell applications, and results in the commercial production of electrochemical cells having high energy density storage, efficient reversibility, high electrical efficiency, bulk hydrogen storage without structural change or poisoning, long cycle life, and deep discharge capability.
Certain hydrogen absorbing alloys result from tailoring the local chemical order and local structural order by the incorporation of selected modifier elements into a host matrix. Disordered hydrogen absorbing alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
The use of disordered negative electrode metal hydride material significantly increases the reversible hydrogen storage characteristics required for efficient and economical battery applications, and results in the commercial production of batteries having high energy density storage, efficient reversibility, high electrical efficiency, bulk hydrogen storage without structural change or poisoning, long cycle life, and deep discharge capability.
Some extremely efficient electrochemical hydrogen storage alloys were formulated, based on the disordered materials described above. These are the Ti—V—Zr—Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 (“the '400 Patent”) the disclosure of which is incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti—V—Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C14 and C15 type crystal structures.
Other Ti—V—Zr—Ni alloys, also used for rechargeable hydrogen storage negative electrodes, are described in U.S. Pat. No. 4,728,586 (“the '586 Patent”), the contents of which is incorporated herein by reference. The '586 Patent describes a specific sub-class of Ti—V—Ni—Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them. Other hydrogen absorbing alloy materials are discussed in U.S. Pat. Nos. 5,096,667, 5,135,589, 5,277,999, 5,238,756, 5,407,761, and 5,536,591, the contents of which are incorporated herein by reference.
The positive electrodes of a Ni—MH battery cell include a nickel hydroxide material as the active electrode material. Generally, any nickel hydroxide material may be used. The nickel hydroxide material used may be a disordered material. The use of disordered materials allow for permanent alteration of the properties of the material by engineering the local and intermediate range order. The general principles are discussed in U.S. Pat. No. 5,348,822, the contents of which are incorporated by reference herein. The nickel hydroxide material may be compositionally disordered. “Compositionally disordered” as used herein is specifically defined to mean that this material contains at least one compositional modifier and/or a chemical modifier. Also, the nickel hydroxide material may also be structurally disordered. “Structurally disordered” as used herein is specifically defined to mean that the material has a conductive surface and filamentous regions of higher conductivity, and further, that the material has multiple or mixed phases where alpha, beta, and gamma-phase regions may exist individually or in combination.
The nickel hydroxide material may comprise a compositionally and structurally disordered multiphase nickel hydroxide host matrix which includes at least one modifier chosen from the group consisting of Al, Ba, Bi, Ca, Co, Cr, Cu, F, Fe, In, K, La, Li, Mg, Mn, Na, Nd, Pb, Pr, Ru, Sb, Sc, Se, Sn, Sr, Te, Ti, Y, and Zn. The nickel hydroxide material may include a compositionally and structurally disordered multiphase nickel hydroxide host matrix which includes at least three modifiers chosen from the group consisting of Al, Ba, Bi, Ca, Co, Cr, Cu, F, Fe, In, K, La, Li, Mg, Mn, Na, Nd, Pb, Pr, Ru, Sb, Sc, Se, Sn, Sr, Te, Ti, Y, and Zn. These embodiments are discussed in detail in commonly assigned U.S. Pat. No. 5,637,423 the contents of which is incorporated by reference herein.
The nickel hydroxide materials may be multiphase polycrystalline materials having at least one gamma-phase that contain compositional modifiers or combinations of compositional and chemical modifiers that promote the multiphase structure and the presence of gamma-phase materials. These compositional modifiers are chosen from the group consisting of Al, Bi, Co, Cr, Cu, Fe, In, LaH3, Mg, Mn, Ru, Sb, Sn, TiH2, TiO, Zn. Preferably, at least three compositional modifiers are used. The nickel hydroxide materials may include the non-substitutional incorporation of at least one chemical modifier around the plates of the material. The phrase “non-substitutional incorporation around the plates”, as used herein means the incorporation into interlamellar sites or at edges of plates. These chemical modifiers are preferably chosen from the group consisting of Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn.
The nickel hydroxide material may comprise a solid solution nickel hydroxide material having a multiphase structure that comprises at least one polycrystalline gamma-phase including a polycrystalline gamma-phase unit cell comprising spacedly disposed plates with at least one chemical modifier incorporated around the plates. The plates may have a range of stable intersheet distances corresponding to a 2+ oxidation state and a 3.5+, or greater, oxidation state. The nickel hydroxide material may include at least three compositional modifiers incorporated into the solid solution nickel hydroxide material to promote the multiphase structure. This embodiment is fully described in U.S. Pat. No. 5,348,822, the contents of which is incorporated by reference herein.
Preferably, one of the chemical modifiers is chosen from the group consisting of Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn. The compositional modifiers may be chosen from the group consisting of a metal, a metallic oxide, a metallic oxide alloy, a metal hydride, and a metal hydride alloy. Preferably, the compositional modifiers are chosen from the group consisting of Al, Bi, Co, Cr, Cu, Fe, In, LaH3, Mn, Ru, Sb, Sn, TiH2, TiO, and Zn. In one embodiment, one of the compositional modifiers is chosen from the group consisting of Al, Bi, Co, Cr, Cu, Fe, In, LaH3, Mn, Ru, Sb, Sn, TiH2, TiO, and Zn. In another embodiment, one of the compositional modifiers is Co. In an alternate embodiment, two of the compositional modifiers are Co and Zn. The nickel hydroxide material may contain 5 to 30 atomic percent, and preferable 10 to 20 atomic percent, of the compositional or chemical modifiers described above.
The disordered nickel hydroxide electrode materials may include at least one structure selected from the group consisting of (i) amorphous; (ii) microcrystalline; (iii) polycrystalline lacking long range compositional order; and (iv) any combination of these amorphous, microcrystalline, or polycrystalline structures.
Also, the nickel hydroxide material may be a structurally disordered material comprising multiple or mixed phases where alpha, beta, and gamma-phase region may exist individually or in combination and where the nickel hydroxide has a conductive surface and filamentous regions of higher conductivity.
Nickel-metal hydride batteries are used in many different applications. For example, nickel-metal hydride batteries are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are also used in many different vehicle applications. For example, nickel-metal hydride batteries are used to drive both pure electric vehicles (EV) as well as hybrid electric vehicles (HEV). Hybrid electric vehicles utilize the combination of a combustion engine and an electric motor driven from a battery.
There are different requirements for the energy storage system of a hybrid electric vehicle than for a pure electric vehicle. Range is a critical factor for a practical pure electric vehicle, making energy density the critical evaluation parameter. In contrast, in hybrid electric vehicle applications, gravimetric and volumetric power density is the overwhelming consideration. A key enabling requirement for HEV systems is an energy storage system capable of providing very high peak power combined with high energy density while at the same time accepting high regenerative braking currents at very high efficiency.
The output power of a battery may be increased by reducing the battery's internal resistance. The internal resistance of the battery includes the resistance of both the positive and negative electrodes, the resistance of the electrolyte, separators as well as other components. The resistance of the electrodes may be decreased by lowering the resistance of the electrode components such as the electrode tabs, electrode substrates as well as the resistance of the electrode active compositions. The resistance of the electrode substrates may be lowered by using more conductive materials (such as copper) wherever possible. The resistance of the positive and/or negative active electrode compositions may be lowered by adding conductive additives to the active material. For example, conductive additives (such as nickel, graphite and carbon particles) may be mixed together with the active electrode materials to form an active electrode composition having an increased conductivity. The methods discussed above, while lowering the resistance and increasing the power of the electrodes have still not realized the full potential thereof. There is still a need for significant gains in power. Therefore, there is a need in the art for additional improvements in the conductivity for both the positive and negative electrodes.